Method of testing a microprocessor by masking of an internal clock signal

ABSTRACT

An apparatus and method for controlling the stopping of the clock signal utilized by the processing unit of a computer system comprises the use of a novel external pin which can be enabled to initiate a sequence of events that results in the halting of the internal clock signal. The invention includes a microcode engine that responds to the assertion of the external pin by executing a sequence of steps which stops the current instruction on an instruction boundary. A logic circuit then generates a signal that masks the clock signal produced by the system&#39;s phase-locked loop. An interrupt mechanism is also utilized to prioritize the occurrence of the external signal among other system interrupts. The interrupt mechanism insures that the processor never has its clock stopped in the middle of a bus cycle.

This is a continuation of application Ser. No. 07/970,576, filed Nov. 3,1992 now U.S. Pat. No. 5,473,767.

FIELD OF THE INVENTION

The present invention relates to the field of computers and computersystems. More specifically, the invention relates to featuresincorporated within a computer system or within a microprocessor forcontrolling clocking signals.

BACKGROUND OF THE INVENTION

The related technologies of computer and microcomputer design have madeincredible advances in the past several decades. New architectural ideascombined with advances in integrated circuit technology have created aseries of machines which achieve remarkable performance results.

One way that computer architects and designers can further improve theperformance of their machines is by creating an apparatus or mechanismfor stopping a processor regardless of the current instruction it isexecuted. Stopping a processor's operation is useful, for example, whentesting the state of the processor. One of the problems that has plaguedprevious technologies is that a processor's state could only be tested,and therefore guaranteed to be valid, when the internal clock signal wasstopped and certain predetermined conditions were satisfied. By way ofexample, such conditions usually occurred during a HALT state or duringan input/output (I/O) read. Under these conditions, the processor'sstate was known, so that the processor could be tested at that point.The problem with this prior art approach, however, is obvious; namely,that testing a processor only when a certain number of predeterminedconditions are satisfied is highly impractical.

The alternative to stopping a processor on predetermined conditions isto stop the processor asynchronously by disabling theexternally-generated signal used as a reference to generate the internalclock rate of the device. The drawback to stopping the processorasynchronously in this manner is that the processing unit is very oftenin the middle of executing one of its instruction. In the middle of aninstruction or micro-instruction, both internal and external buses areprecharged. Stopping the processor in the middle of a bus cycledischarges the bus, with a resultant loss of the information beingtransferred.

Another problem with asynchronously disabling the external referencefrequency generator involves the fact that most microprocessors andcomputer systems utilize a phase-locked loop (PLL) circuit to multiplythe reference frequency by some factor to generate the system's internalclock rate. The internal clock signal is utilized by the centralprocessing unit (CPU) of the computer during the execution of itsvarious functions and instructions. The problem arises that if the clockis stopped externally, then the internal phase-locked loop circuitry islikewise disabled. Under such circumstances, re-enabling the externalreference frequency does not produce an instantaneous response from thePLL; that is, the PLL requires some fixed time pedod (e.g., severalhundred milliseconds) to stabilize. During this start-up time period,spurious signals and glitches are commonly generated, leading tounpredictable results. Thus, starting and stopping of the processor'sclock by disabling the external reference input frequency results in aloss of psuedo-instantaneous response.

What is needed then is a means for stopping the CPU's clock at any time,regardless of the instruction that the processor is presently executing,while guaranteeing that the processor is in a known state. As will beseen, the present invention allows the user to stop the clock of aprocessor within a computer system asynchronously, while stillguaranteeing that the state of the processor is preserved. Guaranteeingthe processor's environment just previous to stopping the processor'sclock allows the device to be tested in a manufacturing environment.Another advantage of the present invention is that the processor can bere-enabled without having to restart the system's PLL circuitry. Thisobviates the need to otherwise wait an inordinate length of time for astable clock signal to be generated.

SUMMARY OF THE INVENTION

A computer system which includes a processor is described. The inventioncovers an apparatus and method of controlling the stopping of the clockutilized by a central processing unit (CPU) of a computer system. Whilestopping the clock, the invention also guarantees the state of theprocessor. In one embodiment, the invention comprises the use of a novelexternal pin which can be enabled to initiate a sequence of events thatresults in the halting of the internal clock signal coupled to the CPU.

In an exemplary, the invention includes a microcode engine coupled toreceive the signal provided by the external pin. In response, themicrocode engine then executes a sequence of steps which stops theexecution of the current instruction on an instruction boundary. Theexternal pin is also coupled to a logic circuit which generates a signalthat masks the CPU's clock. An interrupt mechanism is also utilized toprioritize the occurrence of the external stop₋₋ clock signal amongother system interrupts. The interrupt mechanism insures that theprocessor does not stop its clock in the middle of a bus cycle.

One of the added benefits of the present invention is that it may beutilized to emulate the division of the clock (e.g., divide by 2,4 etc.)by throttling the clock signal on for a given time, and then off foranother time period. This has a number of advantages. First, the PLLthat generates the internal clock does not need to be modified to allowthe division of clock cycles. This means that the present inventionallows stopping and starting of the clock function without alteration ofthe system's phase-lock loop circuitry.

Furthermore, the present invention makes bus designs across multipletime domains much easier; that is, going from the local CPU bus to astandard frequency bus (e.g., like an 8 Mhz ISA bus) is greatlysimplified. This result is achieved because of two primary reasons.First, because the clock is stopped on instruction boundaries, buscycles are never accidentally extended. This means that slave logiccoupled to the processor's bus need not be designed in a way thatcomprehends master extended bus cycles. The second reason simply has todo with the fact that the processor's bus always operates at the samefrequency. In prior art designs where the processor's clock signal wasdivided, as opposed to being throttled, logic coupled to the processor'sbus had to comprehend not only the processor's maximum frequency, butall potential divided clock frequencies as well. On the other hand,devices coupled to a throttled processor's bus only see bus cyclesoccurring at the maximum frequency.

Another advantage of the invention is that the bus connected to the CPUis made much easier to design since the CPU is not required to have itsclock stopped in the middle of a bus cycle. Guaranteeing the state ofthe processor just prior to stopping the processor's internal clocksignal makes the computer system highly testable in a manufacturingenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription which follows and from the accompanying drawings, whichhowever, should not be taken to limit the invention to the specificembodiments shown, but are for explanation and understanding only.

FIG. 1 illustrates prior art method of stopping the clock signal coupledto a central processing unit of a computer system.

FIG. 2 is a block diagram of one embodiment of the present invention.

FIG. 3 illustrates the relationship of the STPCLK pin to a set ofinstructions being executed within the execution unit of a computersystem is asserted and then deasserted.

FIG. 4 is a timing diagram which illustrates the sequence of eventswhich takes place within the computer system when the inventive STPCLKpin is asserted and then deasserted.

FIG. 5 is a flow chart depicting the sequence of events involved instopping of a processor's internal clock signal.

DETAILED DESCRIPTION

An apparatus and method for stopping the clock signal coupled to acentral processing unit of a computer, regardless of the instruction theprocessing unit is presently executing, is described. In the followingdescription, numerous specific details are set forth such as specificevent types, circuits, instruction types, etc., in order to provide athorough understanding of the present invention. It will be obvious,however, to one skilled in the art that these specific details may notbe needed to practice the present invention.

FIG. 1 illustrates a prevalent prior art technique for stopping orhalting the clock signal coupled to a central processing unit (CPU)within a computer system. By way of example, the scheme illustrated inFIG. 1 is commonly found in such microprocessors as the 80286microprocessor manufactured by Intel Corporation. As shown, anoscillator 11 provides a periodic signal coupled to CPU 10 through ANDgate 12. The output of AND gate 12 represents the external frequencyinput (EFI) provided to CPU 10. The other input of AND gate 12 iscoupled to a signal labeled STOP#, which provides a means for decouplingthe EFI signal from CPU 10. If the STOP# signal is low, then the outputof AND gate 12 will be low regardless of the signal provided byoscillator 11. On the other hand, if the STOP# signal is at a logicalhigh level, then the output of oscillator 11 will be coupled directly toCPU 10.

As discussed above, there exists a variety of problems associated withthe prior art scheme of FIG. 1. For instance, testing the CPU under sucha scheme is difficult since it is virtually impossible to test thestopping of the clock on all combinations of instructions which mightcause the product to fail. Another related problem is that stopping ofthe clock in the manner shown in FIG. 1 does not preserve the state ofany of the buses in the computer system. This means that the clock couldbe stopped in the middle of a bus cycle, in which case the informationassociated with that bus cycle would be irretrievably lost.

With reference now to FIG. 2, a block diagram of the currently preferredembodiment of the present invention is shown. The present invention ispreferably implemented on a single integrated circuit (IC) 20 whichincludes a CPU driven by an internal clock signal, labeled ICLK, on line33. The internal clock signal of the CPU is generated by means of anexternal reference oscillatory signal which drives a phase-lock loop(PLL) circuit 21. PLL 21 multiplies the reference frequency by apredetermined factor to produce a clock signal along line 31, coupled toone input of AND gate 22. Note that line 31 is labeled CLK; it is thissignal which provides the clock signal to the computer's circuits whichcannot tolerate an interruption in the clock signal. For example, aconstant clock signal is often required for various cache operations,snooping logic, hold, hold acknowledge operations and arbitration logic.Functions such as these receive the CLK signal.

In the current embodiment, it is only the ICLK signal which isinterrupted or halted in accordance with the present invention. The ICLKsignal, shown being provided on line 33, clocks the vast majority of CPUoperations, including program and instruction execution. As shown inFIG. 2, it is the ICLK signal that is selectively masked by AND gate 22.This masking operation is achieved by taking line 30 to a logical lowpotential, i.e., active low. Line 30 is labeled as STP₋₋ MY₋₋ CLK and isprovided as an output from the STPCLK logic block 24. The STPCLK logicblock 24 includes an ordinary state machine which is coupled to receivethe CLK signal provided by PLL 21. In addition, STPCLK block 24 receivesinputs from interrupt prioritizer 26 along line 29, from microcodeengine 27, and also from the external STPCLK pin along line 28.Likewise, line 28 is also shown being coupled to microcode engine 27 andinterrupt prioritizer 26.

It should be understood that the present invention represents a distinctchange from the prior art approach in several respects. To begin with,instead of decoupling the external reference frequency signal from theprocessor--as shown in the prior art--the present invention utilizes aninternal mechanism for decoupling the internal clock signal withoutdisturbing the operation of the phase-lock loop. At the same time itguarantees that the clock is stopped in a known state (e.g., on aninstruction boundary).

Note also that the embodiment shown in FIG. 2 includes an interruptprioritizer 26. Prioritizer 26 controls events other than branches thatchange the normal flow of instruction execution. Interrupt prioritizer26 operates by detecting interrupt conditions external to integratedcircuit 20, and then granting a priority status to each of thoseinterrupt events. By way of example, in FIG. 2 prioritizer 26 is shownreceiving a plurality of interrupt signals, including a non-maskableinterrupt signal (NMI). A NMI signal may, for instance, represent aparity error generated by the system logic. Also shown are interruptsignals generated by the system management (SMI), and a RESET interruptsignal.

In the embodiment of FIG. 2, the STPCLK# signal is shown being routed tointerrupt prioritizer 26 as well as to STPCLK logic block 24 andmicrocode engine 27. When the external STPCLK# signal is asserted, itgenerates an interrupt to microcode engine 27. This interrupt will berecognized on the next instruction boundary and is granted a highinterrupt priority status by interrupt prioritizer 26. Microcode engine27 includes a STPCLK microcode entry point and microcode program,designed to implement a number of specialized tasks associated withstopping of the internal clock. By way of example, when the STPCLK#signal is asserted, control is passed to the STPCLK microcode handler onthe very next instruction boundary. The STPCLK microcode then waitsuntil the prefetcher is idle and then sets an internal STPRDY bit. TheSTPRDY bit is shown coupled to STPCLK logic block 24. The STPRDY signalfrom the processor indicates that the ICLK signal is ready to bestopped. Engine 27 then initiates execution of a microcode loop whichexamines the STPCLK micro-flag (i.e., driven by the STPCLK# pin). Whenthe STPCLK# signal becomes inactive, control then falls out of theSTPCLK microcode loop and the processor begins executing the nextinstruction in the sequence of instructions given by the user code.

At this point of the process, the STPCLK logic block 24,--upon seeingboth the STPCLK# and STPRDY signals active,--stops the internal clock tothe CPU core by forcing line 32 to a low logic potential. With the STP₋₋MY₋₋ CLK signal at a logical low potential, the internal clock signal,ICLK, becomes inactive. Finally, when the STPCLK logic block 24recognizes that the STPCLK# signal is externally deactivated, it thenrestarts the clock to the processor by de-asserting the STP₋₋ MY₋₋ CLKsignal.

It will be appreciated by practitioners in the art that the use of aninterrupt prioritizer in the present invention insures that theprocessor will be testable, since every time the clock is stopped theprocessor will be in a known state. Internally, the STPCLK pin istreated as if it were any other interrupt generated by system logic.Externally, of course, the STPCLK# pin appears like any other externalinput to the processor. When the STPCLK# pin is asserted, the CPU haltsits internal clock without interfering with either the externalreference oscillatory signal or the operation of the internal PLL. Whenthe STPCLK# pin is deasserted, the CPU then restarts its internal clock.

It should also be apparent from the above discussion that any user couldassert the STPCLK# pin active so that internally the processor wouldstop its clock. In addition, the STPCLK microcode might includemicro-instructions to generate a bus cycle which would acknowledge thatthe processor is in fact stopping its clock (i.e., an acknowledge buscycle). For example, issuing an acknowledge bus cycle could be importantat the system level.

One of the ways in which the present invention is especially useful isin laptop computer systems in which power management is a primaryconsideration. In such a system, external logic could be used to disablethe internal clock function of the CPU when the computer system was idleor otherwise not in use. When the external logic detects an event thatnormally would wake up the processor, the STPCLK# pin would then bede-asserted so that the processor could then resume operating withoutthe need for a lengthy start-up period.

With reference now to FIG. 3, there is shown an exemplary timing diagramillustrating the relation of the STPCLK# pin to a normal sequence ofinstructions being executed in the execution unit of a microprocessor.Essentially, FIG. 3 illustrates the STPCLK# pin transitioning from alogical high to a logical low level, where a logical low levelrepresents an active state. As soon as the STPCLK# pin transitions low,a STPCLK interrupt signal is generated on the next instruction boundary.At this point, the microcode engine recognizes that a jump to the STPCLKmicrocode program is to occur. This activity is shown occurring in FIG.3 by arrow 40. Note that in FIG. 3, the STPCLK# pin is asserted in themiddle of a MOV instruction; however, the internal clock signal of theprocessor is not halted until the end of the current instruction, i.e.,the instruction boundary before the next STO instruction.

The STPCLK microcode program performs several important functions in thecurrent embodiment. To begin with, all of the pipelines within theprocessor are flushed and then idled. Preferably, the microcode wouldthen indicate to the bus unit to execute a STPCLK acknowledge cycle.After that, the microcode then indicates to the STPCLK logic block 24 tostop the internal clock by asserting the STP₋₋ MY₋₋ CLK signal on line30 (see FIG. 2). At this point, the microcode engine simply waits untilthe STPCLK# pin is deasserted; that is, it simply loops on itself untilthe user or system decides to restart the internal clock.

When the STPCLK# pin is de-asserted, the STPCLK logic block 24automatically restarts the clock by deactivating the STP₋₋ MY₋₋ CLKsignal. During the time that the internal clock signal is halted, PLL 21remains active so that the CLK signal provided on line 31 is alsoactive. After the STPCLK# pin has been deasserted, the microcode enginedetects that the ICLK signal is now active and generates a return.Following the return, the next instruction in the normal sequence ofinstructions can begin execution.

The present invention ensures that the processor is testable because itis always in a known state (i.e., the STPCLK microcode stops executionon an instruction boundary) whenever the ICLK signal is stopped.Furthermore, all pipelines and instruction queues are flushed by themicrocode program. Microcode control also guarantees that execution isnever halted in the middle of a bus cycle. This eliminates anyprecharging problems associated with bus cycles or pipeline stages.Because the STPCLK pin provides control of the ICLK signal at a point inthe circuit beyond the phase-lock loop, this also permits an overdrivestrategy, where in a new processor can be inserted where the PLL isinternally multiplying the clock by some factor (e.g., 2×).

To better understand the operation of the present invention consider theexample of FIG. 4 and the flowchart of FIG. 5. FIG. 4 illustrates thetiming relationship of the sequence of events which typically occurswhenever the internal clock of the processor is to be stopped. In FIG.4, the execution unit (EU) is shown having three current instructions,11,12, and 13. These instructions may be part of a sequence ofinstructions currently being executed by the processor. The STPCLKmicrocode program is shown being run between instructions 12 and 13.

In the example of FIG. 4, the STPCLK# pin is shown being asserted by thehigh-to-low transition 42 which occurs during the middle of instruction12. The assertion of the STPCLK# pin is shown by ellipse 50 in FIG. 5.When the STPCLK# pin is asserted, it sets a microflag in the microcodeengine, and also signals the state machine in the STPCLK logic block.Interrupt prioritizer 26 makes sure that the microcode engine recognizesthis input as an interrupt to be asserted at the next instructionboundary. Note that FIG. 4 illustrates the STPCLK# signal beingrecognized on the instruction boundary following the execution ofinstruction 12. It is appreciated that there is an associated set-uptime between the time that the STPCLK# pin is asserted, and the timethat interrupt prioritizer 26 grants it priority status.

On the instruction boundary following the 12 instruction, the microcodeengine determines that the STPCLK interrupt is pending and jumps to theSTPCLK microcode program. The STPCLK microcode program then makes surethat the CPU is in a known and stable state prior to halting to theinternal clock signal. This is shown occurring in FIG. 5 at decisionblock 51. Once the CPU is in a known state on an instruction boundary,the STPCLK microcode program empties the bus unit of any outstanding buscycles, generates a stop₋₋ clock acknowledge bus cycle, and then emptiesthe internal pipelines. This is shown occurring in FIG. 5 by blocks 52,53 and 54. The STPCLK microcode program also Stops the prefetcher fromprefetching. In a preferred implementation, once all bus activity hasbeen halted, a STPCLK acknowledge bus cycle can be run. The STPCLK ACKbus cycle is shown occurring in FIG. 4 just prior to the bus unit beingdeactivated.

What happens next is that the STPCLK microcode program tells the STPCLKlogic block to assert the STP₋₋ MY₋₋ CLK signal, thereby masking theICLK. FIG. 4 shows the STP₋₋ MY₋₋ CLK # signal going low just after theSTPCLK ACK bus cycle is completed. At the same time, the ICLK signal isshown being deactivated. Once the STP₋₋ MY₋₋ CLK signal has beenasserted, the ICLK signal to the CPU is halted and CPU logic operationends. Assertion of the STP₋₋ MY₋₋ CLK signal is represented in theflowchart of FIG. 5 by block 55.

At this stage, the microcode program simply checks to see whether theSTPCLK# pin is still active (e.g., decision block 55 in FIG. 5). Oncethe STPCLK# pin is de-asserted by external logic, the CPU clock isrestarted and execution of the next instruction (13) is returned. Thisis shown occurring at ellipse 57 in FIG. 5. Arrow 45 in FIG. 4 shows howthe low-to-high transition of the STPCLK# pin initiates the sequence ofevents which results in de-assertion of the STP₋₋ MY₋₋ CLK signal andreactivation of the ICLK to the CPU. After ICLK is activated once again,bus activity also resumes.

Note that with the clock restarted, the microcode engine detects thatthe STPCLK signal has been deasserted and ends the STPCLK microcodesequence so that the CPU can execute the next instruction. If the CPUhad stopped its clock from a HALT state, then it would be necessary toset the CPU's context (i.e., register states, etc.) so that upon leavingthe STPCLK microcode program, the processor would reenter the HALTstate.

Whereas many alternations and modifications to the present inventionwill no doubt become apparent to the person of ordinary skill in the artafter having read the foregoing description, it is to be understood thatthe particular embodiments shown and described by way of illustrationare in no way intended to be limiting. For example, although thisdisclosure has shown a particular list of events that may be included aspart of a microcode program for stopping the internal clock of aprocessor, other means are also possible without detracting from thespirit or scope of the present invention. Therefore, reference to thedetails of the illustrated diagrams is not intended to limit the scopeof the claims which themselves recite only those features regarded asessential to the invention.

We claim:
 1. A method of testing a microprocessor comprising the stepsof:(a) providing a reference oscillatory signal to said microprocessor,said microprocessor generating an internal clock signal therefrom; (b)asserting an external pin of said microprocessor to initiate the routinefirst placing said microprocessor in a known state and then asserting alogic signal which masks said internal clock signal from at least aportion of said microprocessor; (c) retrieving information associatedwith said known state from said microprocessor.
 2. The method of claim 1further comprising the step of:(d) deasserting said external pin torecouple said internal clock signal to said at least a portion of saidmicroprocessor.
 3. The method of claim 2 further comprising the step(e)resuming bus activity associated with said microprocessor following thedeassertion of said external pin.
 4. The method of claim 1 wherein saidroutine further comprises issuing a special bus cycle just prior todeactivating a bus unit of said microprocessor.
 5. The method of claims1, 2, 3, or 4 wherein said reference oscillatory signal is providedcontinuously to said microprocessor.